Using data from the National Longitudinal Survey of Youth 1997, a nationally representative cohort of young adults, the authors analyzed relationship type at the time of a first birth (N = 4,044). More than 10% of births were to a postconception cohabiting household (cohabitations that were initiated between conception and birth), a higher proportion of births than those born to postconception married households. Individuals in postconception and preconception cohabiting relationships (cohabitations that existed prior to conception) were demographically similar; both groups were associated with lower levels of socioeconomic advantage relative to those in preconception and postconception marriage. Postconception and preconception cohabiting relationships were associated with similar levels of dissolution, as 40% dissolved within 3 years of a child's birth. Having a marital union, rather than whether relationship was established pre- or postconception, was more strongly associated with who selected into the relationship and how long the relationship lasted.